In the past, it has been desirable to make life predictions on products made of rubber and other low modulus materials. Such materials may be tested to determine the resistance of the material to cracks or crack propagation. Typically, a strip of the material is placed into a reciprocating device such as a mechanical or servo- hydraulic testing machine. The strip is slit or precracked at an edge thereof and then repeatedly flexed or stretched a predetermined amount, with periodic measurements of the growth of the slit or crack being manually taken. From these measurements, the crack growth rate could be determined.
In the past, the periodic measurements were taken visually by an operator through a microscope or other optical device appropriately equipped with a reticle. The growth of the crack was thereby monitored and recorded as a function of the number of flexing cycles imparted to the sample or specimen. The growth of the crack length as a function of the number of flexing cycles could then be plotted. The derivative of this curve is then the growth rate, from which the resistance of the material to crack growth may be determined in a well known fashion.
In the prior art, the operation has been totally manual, relying upon an operator to physically start or stop the reciprocating device, visually observe the length of the crack in the specimen, determine the number of cycles between measurements, determine the crack growth from the last measurement, and ascertain the crack growth rate. Being totally manual, the prior art approach has been time consuming and given to inaccuracies resulting from the subjective operator readings with their inherent human error. The prior test could not be run continuously without the presence of operators over a number of sequential shifts. When the tests were run discontinuously, the results were suspect due to resultant viscoelastic transient effects.
There have been attempts at automating methods of monitoring crack growth in a specimen. Applicant is aware of U.S. Pat. No. 4,175,447 which teaches the use of reflected light rather than transmitted light to characterize a center cracked specimen. Applicant is concerned with edge-cracked specimens, monitored with transmitted light, and with particular means for following both the apex and mouth of the crack. Such is absent in this reference. Similarly, applicant is aware of British Pat. No. 2,057,124 which uses two-dimensional video mounted on a traveling base which is adapted to move parallel to the crack line. The reference fails, however, to teach a single dimensional stationary monitoring system and similarly fails to follow both the apex and mouth of the crack.
U.S. Pat. No. 4,418,563 is of general interest, but it incorporates a test method using caustics rather than transmitted light and is not particularly adapted for automated crack growth measurement. Pat. No. 3,918,299 presents a method of detecting cracks by utilizing eddy currents rather than optical techniques and, in that regard, is of general interest only. Pat. No. 3,983,745 uses displacement and force transducers to inferentially determine crack length, but is only functional for elastic materials, not the time dependent or viscoelastic materials of concern herein. Similarly, British Pat. No. 2,108,684 uses light reflected off an applied coating for testing cracks in stiff elastic materials and is not adapted for the concept presented herein. Finally, an article by Burnos, et al appearing on page 305 of Ind. Lab (U.S.A.), Vol. 3, No. 2 (Feb. 1972) is of very general interest, teaching the production of maximum sharpness stress concentration notches in cylindrical specimens. The concepts presented in this article are not fatigue related, nor are they for monitoring time dependent crack growth.